Method for manufacturing feed pellets and a plant for use in the implementation of the method

ABSTRACT

A method for manufacturing feed pellets, and a plant for the implementation of this method have been explained. The aim has been to improve the manufacturing of porous pellets, first and foremost to achieve a better control of the porosity of the pellets than the known techniques. The pellets come from a pelletizing machine ( 1 ) into a pellet chamber ( 3 ) which is kept at a pressure lower than the ambient pressure. From the chamber ( 3 ) the pellets are passed through an outlet ( 5 ) having a gate lock body ( 6 ).

BACKGROUND OF THE INVENTION

Feed pellets for fish and animals are manufactured on an industrialscale in a multi-stage process. The porosity or specific gravity ofcompletely formed feed pellets or product may be an important criterionfor quality of several types of food and feed products, including feedpellets for reared fish. The porosity of the product is of importance tothe possibility of adding liquid nutrients which are absorbed into theproduct; the porosity is further of importance to floating capacities ina suitable medium, and it is of importance to the texture criterionslike crispness, mouth sensation and toughness. To pellets of fish feedthe porosity is important with respect to the ability of the pellets toabsorb oil in the production process, and for the floatingcapacity/buoyancy in water on feeding.

Existing methods of manufacturing are hard to control accurately, inorder for the product to have the desired porosity, or sufficientporosity for the products, feed substances, and/or feed pellets toachieve a desired absorption of fat.

For some products it will be important to be able to control theproduction process towards a minimum of expansion in, for examplepellets, whereas the opposite will be the case for other products. Inproducing, among other things, feeds for pets such as for example dogsand cats, and feed for reared fish, this possibility of controlling thedegree of expansion/porosity is essential, because the aim is often toenable addition of as much fat/oil as possible in a subsequentprocessing stage for pellets to be fed to fish. For fish feed thecontrol of its degree of expansion is particularly important becausesuch feed should, in addition, exhibit defined sinking capabilities inwater after its fat/oil absorption.

GENERAL DESCRIPTION OF THE INVENTION

This invention relates to a method for manufacturing feed pellets,whereby moist feed pellets are subjected to negative pressure followedby a drying process, in order to achieve a more porous pellet and alower temperature load.

The invention also related to a plant for use in the implementation ofthe method, the plant generally comprising a pellet chamber, preferablyinterconnected downstream of a pelletizing machine, an extruding devicefor pellets or a similar pellet forming device.

An object of the invention is to provide a method and a plant of theinitially mentioned kinds, for use in the manufacturing of porouspellets, whereby a better control of the porosity of the feed product isobtained as compared to the known techniques.

Another object is to achieve a lower temperature load on the productthrough the processing. Since known methods normally require an extrasupply of energy, such as heat, to achieve increased expansion, theopposite effect of what was normally to be expected has been achieved bymeans of the invention. By the use of negative pressure also in thesubsequent drying process and possibly a deep-frying process, anessentially lower temperature load can be achieved for the product ascompared to conventional methods.

Also, the invention comprises a method whereby the pelletizing iscarried out by a first reduced pressure, whereas the subsequent dryingis implemented at a second reduced pressure.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is a schematic diagram of a pelletizing machine and pelletchamber.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

This invention relates to a method for manufacturing feed pellets,whereby moist feed pellets are subjected to negative pressure followedby a drying process, in order to achieve a more porous pellet and alower temperature load.

The invention also relates to a plant for use in the implementation ofthe method, the plant generally comprising a pellet chamber, preferablyinterconnected downstream of a pelletizing machine, an extruding devicefor pellets or a similar pellet forming device.

Feed pellets for fish and animals are manufactured on an industrialscale in a multi-stage process. The components are mixed to a dough-likebody, which is formed into pellets by high pressure and hightemperature, for example in a so-called extruder, after which thepellets are dried and cooled. As warm pellets, typically holding 100 to140° C., are pressure relieved to ambient pressure, the pellets expandbecause of the release of internal pressure and liquid boils out of thepellets. The expansion results in the pellets having a porous structure.

The pellets are cut into pieces of desired length. Considerableremaining moisture in the expanded pellet is generally undesirable,therefore, the pellets may be dried to obtain a desired level ofquality. Such drying may be done in several ways, and some of themethods should be well known to a person skilled in the art.

The porosity or specific gravity of the completely formed product may bean important criterion for quality of several types of food and feedproducts, including feed pellets for reared fish. The porosity of theproduct is of importance to the possibility of adding liquid nutrientswhich are absorbed into the product; the porosity is further ofimportance to floating capacities in a suitable medium, and it is ofimportance to the texture criterions like crispness, mouth sensation andtoughness. To pellets of fish feed the porosity is important withrespect to the ability of the pellets to absorb oil in the productionprocess, and for the floating capacity/buoyancy in water on feeding.

Existing methods of manufacturing are hard to control accurately, inorder for the product to have the desired porosity or sufficientporosity for the products, feed substances, and/or feed pellets toachieve a desired absorption of fat.

For some products it will be important to be able to control theproduction process towards a minimum of expansion in, for examplepellets, whereas the opposite will be the case for other products. Inproducing, among other things, feeds for pets such as for example dogsand cats, and feed for reared fish, this possibility of controlling thedegree of expansion/porosity is essential, because the aim is often toenable addition of as much fat/oil as possible in a subsequentprocessing stage for pellets to be fed to fish. For fish feed thecontrol of its degree of expansion is particularly important becausesuch feed should, in addition, exhibit defined sinking capacities inwater after its fat/oil absorption.

The most common method of increasing the porosity is to increase themechanical and thermal amount of energy added to the raw materials inthe extruding stage of the manufacturing process. When the initialmixture contains surplus vapour after extrusion, the surplus vapour willexpand and result in greater porosity. It is also possible to supplycompressed gas to the extruder, as disclosed in U.S. Pat. No. 5,587,193.In patent publications WO 9503711 and 9816121 are mentioned means forreducing porosity after the extruding stage by extracting positivepressure and surplus vapour inside the extruder. In U.S. Pat. No.5,527,553 is explained a method, in which the pellets are passeddirectly into a warm oil bath at 107–232° C. and cut into a desiredlength in the oil bath. The degree of expansion of pellets is controlledby changing the oil temperature.

An object of the invention is to provide a method and a plant of theinitially mentioned kinds, for use in the manufacturing of porouspellets, whereby a better control of the porosity of the feed product isobtained as compared to the known techniques.

Another object is to achieve a lower temperature load on the productthrough the processing. Since known methods normally require an extrasupply of energy, such as heat, to achieve increased expansion, theopposite effect of what was normally to be expected has been achieved bymeans of the invention. By the use of negative pressure, also in thesubsequent drying process, and possibly a deep-frying process, anessentially lower temperature load can be achieved for the product ascompared to conventional methods.

In a method of the kind specified initially, this object is realized byproceeding in accordance with the claims, and by a plant of theinitially specified kind for the implementation of the method, beingformed so that it exhibits the features stated in the claims.

According to the invention the procedure is such that the pellet isproduced, discharged by or extruded by a pressure which is lower thanthe ambient pressure, pellets being transferred, after a relativelyshort stay by said reduced pressure, to a drying process.

A plant for the implementation of this method comprises a pellet chamberwhich is interconnected in the plant, downstream of the pelletizingmachine, and the plant excels by said pellet chamber being arranged tobe able to be kept at a lower pressure than the ambient pressure, forexample in the order of 100–800 millibar.

In practice, formation of pellets is normally achieved by extrudingpellets in a manner known in itself, but with the important differenceof the extruder discharging the pellets into said pellet chamber whichworks by reduced pressure. The use of reduced pressure will in thisconnection provide improved cooling, i.e. smaller temperature load onthe feed, and increased evaporation of water binding heat. Pelletssubjected to reduced pressure will also expand more than usual, andincreased evaporation of water contributes to the attainment of a moreporous pellet. The expansion may be adjusted by adjusting the negativepressure. Exposure of extruded pellets to low pressure in the pelletchamber may be of a short duration, in typical cases from a few secondsup to one minute, after which the pellets are passed to a dryingprocess.

The pellet temperature generally drops from about 90 to about 50° C.when the pressure (inside the pellet chamber) is reduced from 1000 to200 millibar. At the same time the pellet becomes more porous after thenegative pressure treatment, as the density (less weight per unit ofvolume) decreases from about 450 to 280 grams per liter of pellets.Pressure lower than 200 millibar may also have a favorable effect on thecontrol of the porosity of the feed pellets.

The table below shows the results obtained in a series of experimentswith extruded fish feed by the use of the method and plant according tothe invention. The results show a marked increase in the pellet diameterand a reduction in the bulk density as a measurement of expansion whenthe pressure inside the pellet chamber is reduced from 1000 millibar to200 millibar. The temperature of the product also decreases by droppingpressure, as a consequence of increased evaporation. The experimentreferred to, is only illustrative and not limiting to the scope of theapplication.

Absolute Pellet Bulk Temperature Evaporation pressure diameter densityof pellets of water (mbar) (mm) (g/l) (° C.) (g/kg of feed) 1000 8.3 46091.2  5 800 9 416 80.5  6 600 9.1 368 70.4 11 300 10 296 59.8 — 200 10.2284 52 15

In the experiments mentioned the period of exposure to negative pressurein the pelletizing chamber was 20 seconds. Experiments with continuousdischarging from the pellet chamber (i.e. exposure duration of less than5 seconds), and an exposure duration of 40 seconds, have showncorresponding results for expansion, as those stated above.

A plant for use in the manufacturing of feed pellets excels, accordingto the invention, by the pellet chamber being arranged to allow itselfto be kept at a lower pressure than the ambient pressure, its outletbeing connected to an oil tank or a drying plant, to which the pellet istransferred, and wherein the oil tank or the drying plant is alsoarranged to be able to maintain a lower pressure than that of thesurroundings.

It has proved convenient to let the subsequent drying process also beimplemented by a pressure which is lower than the ambient pressure. Thisstage of the method is advantageous in that it favors the attainment ofthe object aimed at, but this stage may also be eliminated in theimplementation of the method to achieve a satisfactory result. The sameapplied to the deep-frying process which is implemented by reducedpressure in a tank filled with oil, whereby the deep-frying processconstitutes said subsequent drying treatment. For the rest, the dryingprocess may be implemented in a known manner, for example by drying inair.

Also, the invention comprises a method whereby the pelletizing iscarried out by a first reduced pressure, whereas the subsequent dryingis implemented at a second reduced pressure.

Said first pressure and said second pressure may be identical ordifferent from each other.

As mentioned, reduced temperature will be favorable to temperaturesensitive components, and increased porosity is favorable to improve thecapacity of the pellets to absorb oil, whether the oil is added inconnection with the deep-frying, or the oil is added after the pelletshave been dried in another way (for example by drying in warm air).

The outlet of the pellet chamber may have a rotatable gate lock bodyarranged thereto, enabling formed pellets to be drawn continuously or inbatches, while, at the same time, the negative pressure is maintained.

According to the invention pellets are produced in a pelletizing machineand passed from the pelletizing machine into said pellet chamber whichoperates at reduced pressure. The degree of negative pressure relativeto the atmospheric pressure is adjusted with a view to the desiredexpansion of pellets. This method has turned out to provide anessentially better control of the expansion and porosity of the pellets,than measures which have to be taken in a known manner before or duringpelletizing. The reason is believed to be that in changing singleparameters of the pelletizing process, other parameters are alsoinfluenced, which may provide desired results. This is because thepelletizing process creates physical and chemical structures of the rawmaterials by means of the same measures that control expansion (heat,water and pressure).

One may hypothesize that the same effect as by the invention may beachieved by increasing the pressure during pelletizing, and producingpellets into free air with the same pressure drop as achieved by theinvention. However, such a pelletizing pressure increase does not havethat effect. Pelletizing will normally occur with pressure variations,in for example the extruding process, exceeding 1 atmosphere (about 1000millibar), without significantly affecting expansion and porosity. Inthe production of animal feeds the pressure before pelletizing may bebetween 15 and 40 atmospheres, depending on the choice of raw materialsand desired quality of the final product. Pressure is one, but not themost essential process parameter for adjusting the expansion.

As an explanation of the surprising effect obtained by the applicationof the invention, a more rapid boiling out of water and subsequenttemperature drop are considered to enhance acquisition of a desiredlevel of quality of pellet. The drop in temperature results in thepellet matrix setting, thereby preventing the shrinking effect which isotherwise to be expected.

The pressure within the pellet chamber may be in the pressure range from0 millibar to below atmospheric pressure, and will in typical cases bebetween 100 and 800 millibar.

According to the method of the invention, porous pellets are produced ina known manner, but with the novel feature of pellets being dischargedinto a pellet chamber which is kept at a pressure lower than the ambientpressure, typically in the range from one hundred to eight hundredmillibar.

According to the method of the invention, water may be removed from thepellets, and the pores may be filled with fat in subsequent processingstages.

According to the invention the outlet of pelletizing equipment may havea pellet chamber engaged thereto, which is arranged to be maintained ata lower pressure than the surroundings, and which is provided with agate lock opening so that pellets may be drawn continuously, or inbatches, from the pellet chamber, while the chamber is maintained at areduced pressure.

In the following the invention will be described in further detail bymeans of the claims, and reference is made to the accompanying drawing,in which the single FIGURE shows a schematic side view of a plant forthe manufacturing of pellets.

In the FIGURE of the drawing the reference numeral 1 identifies apelletizing machine with an outlet 2 which opens into a pellet chamber3. The pellet chamber 3 has a first vacuum pump 4 arranged thereto,which is arranged to maintain the air pressure inside the pellet chamber3 at a first desired value, lower than the ambient pressure. At itslower end, the pellet chamber 3 is provided with an outlet 5, in whichthere is positioned a gate lock device 6, so that the low pressure ofthe pellet chamber 3 may be maintained while the pellet is discharged.The gate lock device 6 may be of a rotational type, so that pellets maybe fed continuously out of the pellet chamber 3.

The outlet 5 is connected to an inlet 7 in the upper part of an oil tank8 which is partially filled with oil, which is not shown. The oil tank 8has a second vacuum pump 9 arranged thereto, which is arranged tomaintain the air pressure inside the oil tank 8 at a second desiredvalue, which is lower than the ambient pressure and normally also lowerthan said first desired value of the pellet chamber 3. Further, the oiltank 8 may be provided with a heating element with thermostatic control,possibly an agitator, which is not shown, in order to serve for thedeep-frying of pellets.

1. A method for manufacturing feed pellets having an initial pore volumeand fat content said method comprising: A. extruding pellets from a feedmaterial within a pellet extruder having a discharge nozzle; B. exposingsaid extruded pellets to a first pressure lower than ambient pressureimmediately subsequent to said extruding step, said exposure to saidfirst pressure occurring in a pellet chamber downstream from saiddischarge nozzle, whereby said pellets expand and increase said porevolume; C. drying said pellets exposed to said pressure; and D.subsequently adding oil to said pellets to increase said fat content forsaid pellets.
 2. The method according to claim 1, wherein said addingstep occurs during said drying step.
 3. The method according to claim 1,wherein said pellets are exposed to said first pressure for a period oftime not exceeding one minute, said drying step comprising exposure ofsaid pellets to a second pressure, said second pressure being lower thanambient pressure, said drying step further comprising drying at atemperature below 100 degrees Celsius.
 4. The method according to claim3, wherein said first pressure and said second pressure are differentfrom each other.
 5. The method according to claim 1, said drying stepcomprising an oil bath acting as a deep-frying treatment.
 6. A plant formanufacturing feed pellets said plant comprising: A. a pelletizingmachine constructed and arranged for shaping said pellets; B. a pelletchamber adjacent to and downstream from said pelletizing machine, saidpellet chamber having an outlet, said pellet chamber being constructedand arranged to expose said pellets to a pressure lower than ambientpressure; and C. a tank containing oil, said tank being in communicationwith said outlet, said tank comprising a deep-frying container, saidtank being constructed and arranged to expose said pellets to a secondpressure lower than ambient pressure.
 7. The plant according to claim 6,wherein said pressure is between 100 and 800 millibars.
 8. The plantaccording to claim 6, wherein said second pressure is between 100 and800 millibars.
 9. The plant according to claim 6, wherein said pressureand said second pressure are different from each other.
 10. The plantaccording to claim 6, further comprising a lock body between said pelletchamber and said tank.
 11. The plant according to claim 10, wherein saidlock body rotates, said lock body being constructed and arranged to movepellets out of said pellet chamber.
 12. The plant according to claim 10,said pellet chamber comprising a first vacuum pump, said first vacuumpump constructed and arranged to maintain said pellet chamber at saidpressure lower than said ambient pressure, said tank comprising a secondvacuum pump, said second vacuum pump constructed and arranged tomaintain said second pressure lower than said ambient pressure.
 13. Theplant according to claim 12, wherein said second pressure is differentfrom said pressure.
 14. The plant according to claim 13, wherein saidsecond pressure is lower than said pressure.